Linear ball screw-type actuators are used in many applications to transmit linear motion in performing such operations as opening and closing, raising and lowering, pushing and pulling, advancing and retracting, and positioning various devices. Many such linear actuators include an axially restrained screw supported for motor-driven rotation within a fixed outer tube. A ball nut is carried on the screw, and is formed with an internal helical groove that matches a helical groove formed in the screw to accommodate one or more raceway circuits of recirculating load bearing balls. An inner tube is received in the outer tube and is fixed to the ball nut. The inner tube typically mounts a clevis or other coupling at its free end for connection with the device to be actuated. As the screw turns, the ball nut converts the rotary motion of the screw into linear motion of the inner tube, provided the ball nut is rotationally restrained.
Typically, the inner and outer tube members are fabricated of cylindrical tubing and presently, to my knowledge, there is nothing in the construction of the tube members or nut that acts to restrain the inner tube, and thus the nut, against rotation. In most applications, however, this does not present a problem since the inner tube is restrained against rotation by virtue of its attachment to the device to be actuated. In such applications, therefore, the inner tube will not rotate relative to the outer tube, since the device to which it is attached acts to restrain the inner tube from rotating.
There are, however, some applications in which the member to be actuated is not able to restrain the inner tube and nut against rotation, such as when the inner tube of the actuator is to attach to a flexible chain or cable. Such devices tend to twist or rotate with the inner tube, particularly at the fully extended and retracted positions of the inner tube. Where such rotation is objectionable, the approach in the past has been to select non-lead screw type actuators or to utilize a ball screw actuator of the general type described, but in which the telescoping tube members are fabricated of square rather than cylindrical tubing, adding unnecessarily to the cost, weight, and complexity of the actuator.
One of the principal objectives of the present invention is to provide a mechanism and method of restraining the cylindrical inner tube against rotation relative to the fixed outer tube without relying upon the connection to an external device.